Rotary apertured interferometric lithography (rail)

ABSTRACT

A rotary apertured interferometric lithography (RAIL) system that includes interferometric lithography tools, a mask with a slit preferably with an arc shape, and a rotating stage is disclosed. The RAIL system could create a servo pattern of a recording-head trajectory of a hard disk drive in a master for magnetic-contact printing. The master can could be used to form arrays of sub-micron sized magnetic elements on a magnetic disk media for high-density magnetic recording applications.

FIELD OF INVENTION

The present invention relates to master for creating a magnetic patternon a magnetic media.

BACKGROUND

Magnetic disks and disk drives are conventionally employed for storingdata in magnetizable form. Preferably, one or more disks are rotated ona central axis in combination with data transducing heads positioned inclose proximity to the recording surfaces of the disks and movedgenerally radially with respect thereto. Magnetic disks are usuallyhoused in a magnetic disk unit in a stationary state with a magnetichead having a specific load elastically in contact with and pressedagainst the surface of the disk. Data are written onto and read from arapidly rotating recording disk by means of a magnetic head transducerassembly that flies closely over the surface of the disk. Preferably,each face of each disk will have its own independent head.

Disc drives at their most basic level work on the same mechanicalprinciples as media such as compact discs or even records, however,magnetic disc drives can write and read information much more quicklythan compact discs (or records for that matter!). The specific data isplaced on a rotating platter and information is then read or written viaa head that moves across the platter as it spins. Records do this in ananalog fashion where the disc's grooves pick up various vibrations thatthen translate to audio signals, and compact discs use a laser to pickup and write information optically.

In a magnetic disc drive, however, digital information (expressed ascombinations of “0's” and “1's”) is written on tiny magnetic bits (whichthemselves are made up of many even smaller grains). When a bit iswritten, a magnetic field produced by the disc drive's head orients thebit's magnetization in a particular direction, corresponding to either a0 or 1. The magnetism in the head in essence “flips” the magnetizationin the bit between two stable orientations. In currently produced harddisc drives, longitudinal recording is used. In longitudinal recording,the magnetization in the bits is flipped between lying parallel andanti-parallel to the direction in which the head is moving relative tothe disc.

Newer longitudinal recording methods could allow beyond 100 gigabits persquare inch in density. A great challenge however is maintaining astrong signal-to-noise ratio for the bits recorded on the media. Whenthe bit size is reduced, the signal-to-noise ratio is decreased, makingthe bits more difficult to detect, as well as more difficult to keepstable.

Perpendicular recording could enable one to record bits at a higherdensity than longitudinal recording, because it can produce highermagnetic fields in the recording medium. In perpendicular recording, themagnetization of the disc, instead of lying in the disc's plane as itdoes in longitudinal recording, stands on end perpendicular to the planeof the disc. The bits are then represented as regions of upward ordownward directed magnetization (corresponding to the 1's and 0's of thedigital data).

Increasing areal densities within disc drives is no small task. For thepast few years, technologists have been increasing areal densities inlongitudinal recording at a rate in excess of 100% per year. But it isbecoming more challenging to increase areal densities, and this rate isexpected to eventually slow until new magnetic recording methods aredeveloped.

To continue pushing areal densities in recording and increase overallstorage capacity, the data bits must be made smaller and put closertogether. However, there are limits to how small the bits may be made.If the bit becomes too small, the magnetic energy holding the bit inplace may become so small that thermal energy may cause it todemagnetize over time. This phenomenon is known as superparamagnetism.To avoid superparamagnetic effects, disc media manufacturers have beenincreasing the coercivity (the “field” required to write a bit) of thedisc.

In magnetic disk, “servo sectors” are pre-written to define data tracks.Traditionally, servo-sectors were written by a tool called servo-trackwriter. There is also a method to write servo sectors by means ofmagnetic-contact printing, to which the RAIL invention is related to. Inmagnetic disk media, there is a sector called “servo-sector” where thedisk manufacturer prints data for the operation of the disk using amaster by an imaging process. The servo-sector typically occupies about5-10% of the disk capacity. As the areal density increases it is alsodesirable to decrease the size of the servo-sector. This decrease in thesize of the servo-sector could be brought about by increases in theimaging process, known as patterning, for making the master.

Patterning is an operation that removes specific portions on the surfaceof the master. Photolithography is one of the terms used to identify theoperation of patterning. Other terms used are photomasking, masking,microlithography and interference lithography.

Patterning is one of the important operations in disk mediamanufacturing. The goal of the operation is twofold. First, is to createin and on the master surface a pattern whose dimensions are as close toas the resolution of the images on the master. The pattern dimensionsare referred to as the feature sizes or image sizes of the pattern. Thesecond goal is the correct placement (called alignment or registration)of the pattern on the master. The entire pattern must be correctlyplaced on the master and the individual parts of the pattern must be inthe correct positions relative to each other.

Lithography is a pattern transfer process similar to photography andstenciling. In the field of mastering the servo-patterned media (SPM),laser-beam and electron beam lithography are mature technologies. Thesystem consists of an electron source that produces a small-diameterspot and a “blanker” capable of turning the beam on and off. Theexposure must take place in a vacuum to prevent air molecules frominterfering with the electron beam. The beam passes throughelectrostatic plates capable of directing (or steering) the beam in thex-y direction on the SPM. This system is functionally similar to thebeam steering mechanisms of a television set. Precise direction of thebeam requires that the beam travel in a vacuum chamber in which there isthe electron beam source, support mechanisms, and the substrate beingexposed. Since the pattern required generates from the computer, thereis no mask. The beam is directed to specific positions on the surface bythe deflection subsystem and the beam turned on where a photoresist(also called a resist) is to be exposed. Larger substrates are mountedon an x-y stage and are moved under the beam to achieve full surfaceexposure. This alignment and exposure technique is called directwriting.

The pattern is exposed in the resist by either raster or vectorscanning. Raster scanning is the movement of the electron beamside-to-side and down the wafer. The computer directs the movement andactivates the blanker in the regions where the resist is to be exposed.One drawback to raster scanning is the time required for the beam toscan, since it must travel over the entire surface. In vector scanning,the beam is moved directly to the regions that have to be exposed. Ateach location, small square or rectangular shaped areas are exposed,building up the desired shape of the exposed area.

However, with the x-y stage-based lithography tools, accurate r-θposition control was difficult which led to the development of theelectron-beam recorder with a rotating stage and a linear controller,which provided with an accurate r-θ position control. The r-θ positioncontrolled lithography also made it possible to define small features,determined mainly by the beam-spot size, which is important for making amaster for SPM as the density increases. One requirement for the SPMmaster is an accurate trackpitch control. It is affected by factors suchas vibration, random beam deflection due to disturbances, precision andstability of linear actuator control. Despite extensive engineeringefforts, it is difficult to achieve trackpitch variation (3σ) less than10 nm, which is required for the SPM master for hard disks at 200 kTPI(tracks per inch). The limitation is mainly due to inability to controlthe beam position relative to the wafer (substrate) accurately, affectedby the factors mentioned above. Another disadvantage of the e-beamlithography is the slow throughput.

On the other hand, there exists a technology called interferometriclithography. It can make fine periodic patterns, but it could not makepatterns required for the SPM master, consisting of synchronous fields,position-error-signal (PES) bursts, and so on, in an arc shaperepresenting head movement with a rotary actuator in a hard-disk drive.

SUMMARY OF THE INVENTION

This invention preferably relates to a rotary apertured interferometriclithography (RAIL) system that includes interferometric lithographytools, a mask with a slit, preferably with an arc shape, and a rotatingstage. One embodiment is a rotary apertured interferometric lithography(RAIL) system comprising an interferometric tool, a rotating stage and amask having an aperture that creates a servo pattern in a master formagnetic-contact printing. Preferably, the servo-pattern tracks arecording-head trajectory of a hard disk drive. Another embodiment ofthe RAIL system further comprises a phase shifter that controls aposition of an interference fringe. Preferably, the aperture is anarc-shaped slit. Preferably, the RAIL system comprises a laser beam andthe system forms a trackpitch determined by a wavelength of a laser ofthe laser beam and an incident angle of the laser beam. Preferably, themaster has a feature having a size of less than 0.35 micron and astandard deviation of a period of less than 1 nm.

Another embodiment is a master having a feature having a standarddeviation of a period of less than 1 nm, the master being a master formagnetic-contact printing. Preferably, the master contains aservo-pattern that tracks a recording-head trajectory of a hard diskdrive. Preferably, the feature has a size of less than 0.35 micron.

Yet another embodiment is a method of manufacturing a master comprisingapplying a resist to a substrate, patterning the resist byinterferometric lithography to form a patterned resist, and depositing ametal on the patterned resist, wherein the master has a feature having astandard deviation of a period of less than 1 nm and the master is amaster for magnetic-contact printing. Preferably, the depositing a metalcomprises sputtering depositing a metal layer and subsequentlyelectroplating a metal film on the metal layer. Further preferably, thepatterning the resist comprises exposing the resist to a laser beam anddeveloping the resist. In one embodiment, the patterned resist containsdepressions of different depths.

Another embodiment is a method of forming a servo-sector in a magneticdisk medium comprising contacting a master having a feature having astandard deviation of less than 1 nm to the magnetic disk medium andexposing the master to a magnetic field. Preferably, the exposing themaster to a magnetic field creates a magnetic pattern in a magneticlayer of the magnetic disk medium.

Additional advantages of this invention will become readily apparent tothose skilled in this art from the following detailed description,wherein only the preferred embodiments of this invention is shown anddescribed, simply by way of illustration of the best mode contemplatedfor carrying out this invention. As will be realized, this invention iscapable of other and different embodiments, and its details are capableof modifications in various obvious respects, all without departing fromthis invention. Accordingly, the drawings and description are to beregarded as illustrative in nature and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a side view of an embodiment of the RAIL system.

FIG. 2 shows a top view of an embodiment of the RAIL system.

FIG. 3 shows one embodiment of the resulting patterns formed on aphotoresist by the RAIL system.

FIG. 4 shows another embodiment of the resulting patterns formed on aphotoresist by the RAIL system.

DETAILED DESCRIPTION

This invention differs from prior systems by allowing precise trackpitchdetermined by the wavelength of the laser used and the incident angle oftwo beams, with much better trackpitch variation than the prior systems.It can make patterns required for the master for Contact-print ServoPatterned Media (CSPM).

The method of performing RAIL and calibrating MR head geometry inself-servo writing disc drives is described in the U.S. Pat. No.6,317,285 incorporated herein by reference.

For the master of CSPM, one preferred requirement is the trackpitchvariation, and it is determined by the interference optics and notaffected by the environmental disturbance significantly in the presentinvention.

The present invention has an additional benefit of reducing the timerequired for recording a master, since it can be done in a singlerotation of the rotating stage, as opposed to hundreds of thousands ofrotations required in the prior arts using a LBR or EBR.

Briefly, the RAIL process is the following. The required pattern isfirst formed in reticles or photomasks and transferred into the surfacelayer(s) of master through a photomasking steps. The transfer takesplace in several steps. First, the pattern on the mask is transferredinto a layer of a photoresist spread out on a smooth, solid surface suchas that of a silicon wafer or a glass plate. Photoresist is alight-sensitive material similar to the coating on a regularphotographic film. Exposure of light (or laser) causes changes in itsstructure and properties. In one type, for example the negativephotoresist, the photoresist in the region exposed to the light ischanged from a soluble material to an insoluble material. The solubleportions are removed with chemical solvents (developers) leaving a holein the resist layer corresponding to the pattern on the mask.

The second transfer takes places from the photoresist layer into themaster surface layer as follows. The patterned photoresist layer issputter coated with a layer of metal. Then a metal film such as a nickelfilm is electroplated on the sputter deposited metal layer. The metalfilm is peeled off and it has a topography pattern corresponding to thepattern on the mask. This metal film is used as the master. The masteris laid on a magnetic disk and exposed to a magnetic field to create amagnetic pattern in the magnetic layer of the magnetic medium.

The selection of a photoresist depends on several factors. The primarydriving force is the dimensions required on the master surface. Theresist must first have the capability of producing those dimensions.Beyond that it must also function as an etch barrier during the etchingstep, a function that requires a certain thickness for mechanicalstrength, it must be free of pinholes, which also requires a certainthickness. In the embodiment described above, an etch process is notnecessarily required. However, the master has a certain thickness. Inaddition, it must adhere to the top wafer surface or the pattern will bedistorted, just as a paint stencil will give a sloppy image if it is nottaped tight to the surface. These, along with process latitude and stepcoverage capabilities, are resist performance factors. In the selectionof a resist, one often must make trade-off decisions between the variousperformance factors. The photoresist is one part of a system of chemicalprocesses and equipment that must work together to produce the imageresults and be productive, that is, an acceptable cost of ownership forthe whole patterning process.

Resolution capability: The smallest opening or space that can beproduced in a photoresist layer is generally referred to as itsresolution capability. The smaller the line produced, the better theresolution capability. Resolution capability for a particular resist isreferenced to a particular process including the exposing source anddeveloping process. Changing the other process parameters will alter theinherent resolution capability of the resist. Generally, smaller lineopenings are produced with a thinner resist film thickness. However, aresist layer must be thick enough to function as an and be pinhole-free.The selection of a resist thickness is a trade-off between these twogoals. The capability of a particular resist relative to resolution andthickness is measured by its aspect ratio. The aspect ratio iscalculated as the ratio of the resist thickness to the image opening.Positive resists have a higher aspect ratio compared to negativeresists, which means that for a given image-size opening, the resistlayer can be thicker. The ability of positive resist to resolve asmaller opening is due to the smaller size of the polymer. It is alittle like the requirement of using a smaller brush to paint a thinnerline.

Adhesion capability: In its role as an etch barrier, a photoresist layermust adhere well to the surface layer to faithfully transfer the resistopening into the layer. Lack of adhesion results in distorted images.Resists differ in their ability to adhere to the various surfaces usedin chip fabrication. Within the photomasking process, there are a numberof steps that are specifically included to promote the natural adhesionof the resist to the wafer surface. Negative resists generally have ahigher adhesion capability than positive resists.

Photoresist exposure speed, sensitivity, and exposure source: Theprimary action of a photoresist is the change in structure in responseto an exposing light or radiation. An important process factor is thespeed at which that reaction takes place. The faster the speed, thefaster the wafers can be processed through the masking area. Negativeresists typically require 5 to 15 seconds of exposure time, whilepositive resists take three to four times longer. The sensitivity of aresist relates to the amount of energy required to cause thepolymerization or photosolubilization to occur. Further, sensitivityrelates to the energy associated with specific wavelength of theexposing source. Understanding this property requires a familiarizationwith the nature of the electromagnetic spectrum. Within nature weidentify a number of different types of energy: light, short and longradio waves, x-rays, etc. In reality they are all electromagnetic energy(or radiation) and are differentiated from each other by theirwavelengths, with the shorter wavelength radiation having higherenergies.

Common positive and negative photoresists respond to energies in theultraviolet and deep ultraviolet (DUV) portion of the spectrum. Some aredesigned to respond to particular wavelength peaks within those ranges.Some resists are designed to work with x-rays or electron beams(e-beam). Resist sensitivity, as a parameter, is measured as the amountof energy required to initiate the basic reaction. The units aremilijoules per square centimeter (mJ/cm²). The specific wavelengths theresist reacts to are called the spectral response characteristic of theresist. The peaks in the spectrum are regions (wavelengths) that carryhigher amounts of energy.

Process latitude: While reading the sections on the individual maskingprocess steps, the reader should keep in mind the fact that the goal ofthe overall process is a faithful reproduction of the required imagesize in the wafer layer(s). Every step has an influence on the finalimage size and each of the steps has inherent process variations. Someresists are more tolerant of these variations, that is, they have awider process latitude. The wider the process latitude, the higher theprobability that the images on the wafer will meet the requireddimensional specifications.

Pinholes: Pinholes are microscopically small voids in the resist layer.They are detrimental because they allow to seep through the resist layerand etch small holes in the surface layer. Pinholes come fromparticulate contamination in the environment, the spin process, and fromstructural voids in the resist layer.

The thinner the resist layer, the more pinholes. Therefore, thickerfilms have fewer pinholes but they also make the resolution of smallopenings more difficult. These two factors present one of the classictrade-offs in determining a process resist thickness. One of theprincipal advantages of positive resists is their higher aspect ratio,which allows a thicker resist film and a lower pinhole count for a givenimage size.

Particle and contamination levels: Resists, like other process chemicalsmust meet stringent standards for particle content, sodium and tracemetal contaminants, and water content.

Thermal flow: During the masking process there are two heating steps.The first, called soft bake, evaporates solvents from the resist. Thesecond one, hard bake, takes place after the image has been developed inthe resist layer. The purpose of the hard bake is to increase theadhesion of the resist to the wafer surface. However, the resist, beinga plastic-like material, will soften and flow during the hard bake step.The amount of flow has an important effect on the final image size. Theresist has to maintain its shape and structure during the bake or theprocess design must account for dimensional changes due to thermal flow.

The goal of the process engineer is to achieve as high a baketemperature as possible to maximize adhesion. This temperature islimited by the flow characteristics of the resist. In general, the morestable the thermal flow of the resist, the better it is in the process.

The performance factors outlined above are related to a number ofphysical and chemical properties of the resist. A photoresist is aliquid that is applied to the wafer by a spinning technique. Thethickness of resist left on the wafer is a function of the spin stepparameters and several resist properties: solids content and viscosity.

The surface tension of a resist also influences the outcome at spin.Surface tension is a measure of the attractive forces in the surface ofthe liquid. Liquids with high surface tension flow less readily on aflat surface. It is the surface tension that draws a liquid into aspherical shape on a surface or in a tube.

The optical properties of the resist play a role in the exposuremechanism. One property is refraction or the bending of light as itpasses through a transparent or semitransparent medium. The index ofrefraction is a measurement of the speed of light in a material comparedto its speed in air. It is calculated as the ratio of the reflectingangle to the impinging angle. Preferably for photoresists, the index ofrefraction is close to that of glass, approximately 1.45.

Embodiments of this invention are shown in FIGS. 1 to 4. FIGS. 1 and 2show the side and top views of an embodiment of a RAIL system. In FIG.1, two laser beams interfere and expose a photoresist to form a patternon the photoresist. A beam splitter splits a laser beam into twointerfering beams. The RAIL system also preferably has a phase shifterthat controls a position of an interference fringe. The photoresist isthen chemically developed to create a resist pattern with differentdepths in the pattern.

Interference lithography (IL) is the preferred method for fabricatingperiodic and quasi-periodic patterns that must be spatially coherentover large areas. IL is a conceptually simple process where two coherentbeams interfere to produce a standing wave, which can be recorded in aphotoresist. The spatial-period of the grating can be as fine as halfthe wavelength of the interfering light, allowing for structures on theorder of 100 nm from UV wavelengths, and features as small as 30-40 nmusing a deep UV ArF laser, for example.

In particular, it is preferable to control the flexure of the substrateduring exposure of the resist. For example, a controlled flexure of thesubstrate during exposure can reduce the distortion of the pattern from2 dimensions to 1 dimension as well as reduce the magnitude of thedistortions by about a factor of 5.

For spatial periods of the order of 100 nm, one could use a 193 nmwavelength ArF laser. To compensate for the limited temporal coherenceof the source, one could utilize an achromatic scheme in which thespatial period of the printed grating is dependent only on the period ofthe parent gratings used in the interferometer, regardless of theoptical path or the wavelength and coherence of the source. Thus,gratings and grids produced with such a tool are extremely repeatable. A100 nm-period grid can be produced using achromatic interferometriclithography (AIL) on a photoresist. The RAIL system could use AIL, whichcould be used to produce 50 nm period gratings and grids, or 25 nm linesand spaces using reflection gratings with a 58.4 nm helium dischargesource.

The basic principle is that features in crosstrack direction are definedby interference fringes created by two coherent laser beams. It willcreate a periodic pattern (lines and spacing), the period of which isdetermined by

${period} = \frac{\lambda}{2\; \sin \; \theta}$

where λ is the wavelength of the laser and θ is an angle as shown inFIG. 1. For example, with λ=257 nm and θ=80°, the period can be 130 nm,which is sufficient for 390 kTPI. The period is twice the trackpitch ofthe servo patterns as shown in FIG. 3, wherein the period is measuredfrom the leading edge of one track to the leading edge of anotheradjacent track. However, the period could also be measured from anyother defined location on one track to another similarly definedlocation on an adjacent track. For example, the period could be measuredfrom the trailing edge (or center) of one track to the trailing edge (orcenter) of another adjacent track.

The features in downtrack direction are defined by a slit of the maskconfining the exposed area of the beam spot, and the resolution islimited by the diffraction. The patterns as shown in FIGS. 3 and 4 canbe made by the combination of the blanking of the laser beam and theincremental rotation of the wafer, for example. The downtrack length ofthe features is not necessarily as small as the crosstrack length, andcan be on the order of μm, which is sufficient for the master forprinted-pattern assisted self-servo write.

Shifting the phase of one of the laser beams can make thecheckerboard-like patterns as shown in FIG. 3, such that the positionsof the constructive and destructive interference change as desired. Onlyone beam with double intensity, for example, could also be used to makesynchronous field patterns. For a printed-pattern assisted self-servowrite master, it is not necessary to record gray code fields.Preferably, the shape of the slit can be an arc to mimic the headtrajectory in a drive with a rotary actuator.

The master has a substantially uniform period of the pattern with astandard deviation of the period being less than 1 nm, preferably lessthan 0.5 nm. Current e-beam method results in a standard deviation ofthe period of about 3-5 nm. The present invention uses an interferencemethod to create the pattern, which will give a standard deviation ofthe period of less than 1 nm, preferably, less than 0.1 nm. Also, thepattern by the RAIL system has a feature size of less then 0.35 microns,preferably less than 0.3 microns, and more preferably less than 0.25microns. Current photolithography methods result in a feature size ofmore than 0.35 microns.

This application discloses several numerical ranges in the text andfigures. The numerical ranges disclosed inherently support any range orvalue within the disclosed numerical ranges even though a precise rangelimitation is not stated verbatim in the specification because thisinvention can be practiced throughout the disclosed numerical ranges.

The above description is presented to enable a person skilled in the artto make and use the invention, and is provided in the context of aparticular application and its requirements. Various modifications tothe preferred embodiments will be readily apparent to those skilled inthe art, and the generic principles defined herein may be applied toother embodiments and applications without departing from the spirit andscope of the invention. Thus, this invention is not intended to belimited to the embodiments shown, but is to be accorded the widest scopeconsistent with the principles and features disclosed herein. Finally,the entire disclosure of the patents and publications referred in thisapplication are hereby incorporated herein by reference.

1-19. (canceled)
 20. A method of manufacturing a master comprisingapplying a resist to a substrate, patterning the resist byinterferometric lithography to form a patterned resist, and depositing ametal on the patterned resist, wherein the master has a feature having astandard deviation of a period of the feature of less than 1 nm and themaster is a master for magnetic-contact printing.
 21. The method ofclaim 20, wherein the depositing a metal comprises sputtering depositinga metal layer and subsequently electroplating a metal film on the metallayer.
 22. The method of claim 20, wherein the patterning the resistcomprises exposing the resist to a laser beam and developing the resist.23. The method of claim 20, wherein the patterned resist containsdepressions of different depths.
 24. The method of claim 20, wherein thefeature has a size of less than 0.35 micron.
 25. The method of claim 20,wherein the patterning the resist by interferometric lithography is doneby a rotary apertured interferometric lithography (RAIL) systemcomprising an interferometric tool, a rotating stage, at least twointerfering laser beams and a mask having an aperture that creates aservo pattern in a master for magnetic-contact printing, wherein themaster has a feature having a size of less than 0.35 micron and astandard deviation of a period of the servo pattern is less than 1 nm,wherein the aperture is an arc-shaped slit, wherein the chord of thearc-shaped slit extends substantially radially from near the center ofthe rotating stage to near the perimeter of the rotating stage.
 26. Themethod of claim 25, wherein the servo-pattern tracks a recording-headtrajectory of a hard disk drive.
 27. The method of claim 25, furthercomprising a phase shifter that controls a position of an interferencefringe.
 28. The method of claim 25, wherein the system can form theservo pattern with a trackpitch determined by a wavelength of the twolaser beams and an incident angle of the laser beams.
 29. The method ofclaim 25, wherein a beam splitter splits a laser beam into the twointerfering laser beams.
 30. The method of claim 25, wherein the twolaser beams are produced by a 193 nm wavelength ArF laser.
 31. Themethod of claim 25, wherein the interferometric tool comprises anachromatic interferometric lithography (AIL) tool.
 32. The method ofclaim 25, wherein the AIL tool is adapted to produce a servo patternhaving 50 nm period gratings.
 33. The method of claim 25, wherein theservo pattern has a period that is determined by${period} = \frac{\lambda}{2\; \sin \; \theta}$ where λ is thewavelength of the laser and θ is an angle between the normal to theplane of the surface of the rotating stage and one of the twointerfering beams.
 34. The method of claim 25, further comprising awafer with a photoresist.
 35. The method of claim 25, wherein the servopattern is a checkerboard pattern.
 36. The method of claim 25, whereinthe standard deviation of the period of the servo pattern is less than0.5 nm.
 37. The method of claim 25, wherein the feature has a size ofless than 0.25 micron.